This invention relates to an electromagnetic shielding sheet.
Due to the increasing wide use of electrical and electronic instruments, synthetic resin (plastic) materials have been used to make the cases for the instruments because of the resulting benefits in price and weight. However, the problem of leakage of electromagnetic waves emitted from instruments becomes greater year by year. Further, when precise electronic instruments such as computers and measuring devices are exposed to outside electromagnetic waves or radiations, erroneous operation or erroneous measurements may occur. Consequently, governments have established regulations for electromagnetic waves both in West Germany and United States of America, and such regulations are expected in the near future in Japan.
Various methods for shielding instruments from exposure to or from emitting electromagnetic waves have been proposed, but such methods have not been entirely satisfactory. For example, painting the cases of the instruments with conductive paints, or spraying them with melted zinc have been tried. Also plating, vapor depositing or sputtering shielding substances on the surfaces of the plastic casings of the instruments have been used to shield the electrical components. However, they have generally been ineffective for electromagnetic waves of low frequency, and there is also the problem of the shielding materials peeling off.
Stainless steel fibers have also been used to shield electromagnetic waves by mixing the metal fibers with the plastic materials used to make the cases for the instruments. However, this has not been entirely satisfactory either, because it is necessary to add considerable amounts of metal fibers into the plastic to obtain the desired shielding effect and this tends to degrade the strength of the plastic. It also makes it necessary to paint the instruments to cover over for the metal fibers appearing on its surface.
Iron foils have also been used as a shielding material but, an insufficient shielding effect is obtained with magnetic waves of low frequency. Also, when compared with copper and aluminum, the electric resistance is greater with waves of high frequency and the shielding characteristics are not good.
Therefore an object of this invention is to provide an electromagnetic shielding sheet for shielding electrical instruments from electromagnetic waves of both low and high frequency. Further, an object of this invention is to provide an electromagnetic shielding sheet which is flexible and easy to use.